1. Field of the Invention
The present invention relates to a Ni-base alloy suitable for joint with a ferritic steel, a high-temperature member for a steam turbine and a welded rotor for a turbine using the Ni-base alloy and a method for manufacturing the same.
2. Description of Related Art
A Ni-base heat resistant alloy is used for a high-temperature member such as an industrial gas turbine, an aviation jet engine or the like. Since the Ni-base heat resistant alloy contains solid solution strengthening elements such as W, Mo and Co or precipitation strengthening elements such as Al, Ti, Nb and Ta abundantly, it shows an excellent high-temperature strength. Because a γ′ Ni3 (Al, Ti) phase that is a primary strengthening phase has a property of increasing its strength as the temperature rises, it is extremely effective in improving the strength property at a high temperature, and therefore the development of the alloy is carried out mainly with an aim of increasingly precipitating the γ′ phase.
In the meantime, a high-Cr ferritic heat resistant steel has heretofore been used as a steam turbine member used for coal fired power generation. The ferritic heat resistant steel is generally better than a Ni-base alloy in manufacturability and can be used for manufacturing a large forged material, such as a turbine rotor having a weight exceeding 20 tons. With a Ni-base alloy in contrast, it is difficult to manufacture a large material comparable with a ferritic heat resistant steel by current technologies. From the viewpoints of CO2 reduction, resource savings and others, it is required to raise a steam temperature in order to realize electric power generation of a higher efficiency. The heatproof temperature of a currently used ferritic heat resistant steel is about 600° C. and it is necessary to use a Ni-base heat resistant alloy having a higher heatproof temperature in order to further raise the steam temperature.
Since it is difficult to manufacture a large material comparable with a ferritic heat resistant steel with a Ni-base alloy as stated above however, a material of a structure wherein only a part heated to a highest temperature and exposed to severe environment is made of a Ni-base alloy and the other part is made of a conventional ferritic heat resistant steel is proposed.
Such a structure is required to join a Ni-base alloy to a ferritic heat resistant steel by such a means as welding. Problems caused by the difference in the properties between both the materials are concerned therefore and inventions aimed at solving the problems are reported. A Ni-base alloy having a low thermal expansion coefficient aimed at mitigating the thermal stress caused by the difference of thermal expansion coefficient is proposed in Document 1 (Japanese Patent Laid-open No. Hei 09-157779) and Document 2 (Japanese Patent Laid-open No. 2000-256770).
With regard to welding for forming a joining structure, in general, welding of a Ni-base alloy is more difficult than welding of a ferritic heat resistant steel. In the case of a Ni-base alloy, a γ′ phase is precipitated as a strengthening phase by applying solution treatment at 1,000° C. to 1,100° C. and thereafter applying aging treatment at about 750° C. to 1,000° C. However, when welding is applied in the state of precipitating the γ′ phase, cracking is likely to be caused by thermal stress. Consequently, it is desirable to carry out welding operation in the state of minimizing the precipitation of the γ′ phase. When heat treatment of 700° C. or higher is applied to a ferritic heat resistant steel however, strength is extremely hindered and hence temperature of the heat treatment after welding is restricted to about 600° C. to 700° C. Consequently, although the joining structure of a high strength can be realized if a γ′ phase can be precipitated abundantly by the heat treatment of about 600° C. to 700° C., with a conventional Ni-base alloy, the quantity of the precipitated γ′ phase is small and a sufficient strength is not obtained in the temperature range.
Further, when a ferritic steel and a Ni-base superalloy are welded to each other, a normal method is to weld the ferritic steel in the state of being subjected to quenching and tempering and the Ni-base superalloy in the state of being subjected to solution treatment. It is necessary to apply intermediate aging treatment at a temperature of 800° C. to 850° C. for securing ductility and toughness of the superalloy after the welding. But the strength property deteriorates considerably when the ferritic steel part is heated in the temperature range. Furthermore, it is impossible to obtain excellent ductility and toughness at the Ni-base superalloy part when aging treatment and residual stress relaxation treatment are applied at a temperature of 700° C. or lower after the welding.
Document 3 (Japanese Patent Laid-open No. 2005-121023) describes a different-materials-welded rotor comprising a ferritic steel and a precipitation strengthening type Ni-base alloy and having an intermediate ring comprising a solid solution strengthening type Ni-base alloy (IN617) excellent in weldability as an intermediate layer.
An object of the present invention is to provide a Ni-base alloy that is excellent in weldability and low-temperature aging property, is suitable for joint with a ferritic steel and makes it possible to realize a highly reliable welded joining structure with the ferritic heat resistant steel, and a high-temperature member for a steam turbine using such a Ni-base alloy.
Another object of the present invention is to solve a problem of heat treatment consistency that arises when a precipitation strengthening type Ni-base superalloy having a heatproof temperature of 675° C. or higher is joined to a ferritic steel, and to simultaneously render strength, ductility and toughness to a whole welded structure.